1. Field of the Invention
The present invention relates to a sheet loading device mounted on an image forming apparatus such as a copying machine, a printer, a facsimile or the like, and particularly to a sheet loading device for separating and feeding sheets one by one from sheet containing means containing a plurality of sheets loaded therein. Transfer paper, photosensitive paper, thermosensitive paper, electrostatic recording paper, printing paper, originals, cards, cut sheets such as envelopes and so on can be used as sheets, and the material of the sheets is not limited to paper.
2. Description of the Related Art
A conventional image forming apparatus such as a copying machine, a facsimile or the like is equipped with a sheet loading device which contains a plurality of sheets loaded in a container, and which is detachably fitted to the body of the apparatus. When the sheet loading device uses a housing-like feeding cassette, an intermediate plate that serves as sheet loading means for loading sheets is provided at the bottom of the cassette. The intermediate plate is provided so as to be vertically rotatable around an axis provided at the rear end thereof. The intermediate plate is also upwardly urged from the lower side thereof by pressure means such as a coil spring so as to press a stack of sheets loaded on the intermediate plate with a predetermined pressure against feeding means such as a feeding roller. The feeding means is driven to separate and feed the sheets one by one to the next processing area.
In the feeding operation, a claw separation method is generally widely used for preventing multiple feeding of sheets. In this method, separation claws are respectively provided at both edges of the cassette on the front end thereof in the direction of feeding of a stack of sheets. In feeding a sheet, the top sheet is separated from the other sheets by the operation of passing the top sheet over the claws while forming a small flexural loop at the leading end thereof to feed only one sheet.
FIG. 12 illustrates a principal portion of a feeding cassette which employs the claw separation method. In FIG. 12, reference numeral 51 denotes an intermediate plate for loading sheets thereon, and reference numeral 52 denotes springs for upwardly urging the intermediate plate 51 from the lower side thereof. Sheets P (cut sheets or paper) having a predetermined size are loaded on the intermediate plate 51. Reference numeral 53 denotes feeding rollers which press the upper surface of a stack of the sheets loaded on the intermediate plate 51 at a position near the leading ends of the sheets in the direction of feeding due to the lifting force of the springs 52. Reference numeral 54 denotes separation claws provided at the right and left corners on the upper surface of the feeding cassette at the leading end thereof in the direction of feeding of the sheets. The separation claws 54 are respectively mounted so as to be vertically rotatable around shafts 54a and gravitationally fitted to both corners at the leading end of the sheets.
When the feeding rollers 53 are rotated in the direction of arrows R, the top sheet P of a stack of the sheets is sent in the direction of the arrows X by frictional force. At this time, although the top sheet P is apt to move forward, both corners at the leading end thereof collide with the separation claws 54 to inhibit the movement of the sheet P. As a result, a flexure A is formed in the top sheet p between the separation claws 54 and the rollers 53 due to rotation of the feeding rollers 53.
If the size of the flexure A is greater than a given level, both corners of the sheet P, which are held by the separation claws 54, naturally escape from the lower sides of the separation claws 54 to the upper sides thereof by the repulsive force that tends to return the sheet P to the original form, and pass over the separation claws 54. Namely, only the top sheet P of a stack of the sheets is released from the restriction by the claws 54 by formation of the flexure A.
In such a claw separation method, the amount of engagement between the sheets P and the separation claws 54 must be kept constant for maintaining high separation performance. In other words, the amount of engagement between the sheets P and the separation claws 54 is constant during the time from loading of the sheets P to feeding of the last sheet P regardless of the number of sheets P contained in the cassette.
However, demands for increasing the sheet capacity have recently increased because of an increase in the typical volume of communication conducted, and thus attempts have vigorously been made to increase the capacity of the cassette. The cassette case thus has a shape increased in depth. In such a feeding cassette, the rotational axis of the intermediate plate is provided at substantially the center of the depth of the cassette in consideration of the condition that the forward and backward movement of the intermediate plate caused by rotation of the front end thereof is suppressed as much as possible. However, the range of rotation of the intermediate plate becomes wider because of the large depth of the cassette, and thus it is difficult to keep the amount of engagement between the sheets and the separation claws constant.
FIGS. 13(a) to (c) illustrate full loading, medium loading and small loading states, respectively, of the sheets P in the cassette where the amount of engagement between the sheets P and the separation claws 54 is kept constant. Reference characters Pu(a), Pu(b) and Pu(c) indicate the trailing ends of the sheets in the cassette in the full loading, medium loading and small loading states, respectively. In a small-capacity cassette, a method is generally employed in which trailing end regulating means such as a trailing end regulating plate for regulating the position of the trailing ends of the sheets is fixed to the cassette case. When this method is applied to a large-capacity cassette, the trailing end regulating plate must be fixed at a sheet trailing end position b of the sheets in the full loading state, and a gap a thus occurs between the position of the sheet trailing ends of the sheets in the middle loading and small loading states, as shown in FIGS. 13(b) and (c), respectively. In addition, since the range of rotation of the intermediate plate 51 becomes wider, the inclination of the intermediate plate 51 in the small loading state of the sheets is increased, and the sheets P slip downward to the side of the trailing ends in an amount of the gap a. In a cassette having a capacity of about 500 sheets, the maximum gap a is about 5 to 6 mm. Thus, the amount of engagement between the sheets P and the separation claws 54 at the leading ends of the sheets cannot be uniformly maintained, thereby causing poor separation and thus the possibility of deterioration in separation performance.
The techniques proposed for solving the above problems are as follows:
(1) A technique in which a trailing end regulating plate 56 is rotatably pivoted to the bottom of a cassette case 55 containing the intermediate plate 51 so as to pass through a cut hole 51a formed in the intermediate plate 51, as shown in FIG. 14(a). When the intermediate plate 51 rotates around the rotational axis 51b in accordance with the amount of the sheets loaded, one widthwise side of the cut hole 51a is accompanied by the trailing end regulating plate 56, thereby preventing deviation from occurring between the respective sheets due to a change in the amount of sheets P loaded.
(2) Another technique in which a trailing end regulating plate 57 is provided so as to be slidable in the lengthwise direction of the sheets P contained in the cassette 58, as shown in FIG. 15(a). When an intermediate plate 59 is upwardly rotated around a rotational axis 59a by pressure means (not shown), the intermediate plate 59 is connected to the trailing end regulating plate 57 by a connecting member 60 to slide the trailing end regulating plate 57 in linkage with the rotation of the intermediate plate 59, thereby preventing the gap produced at the trailing ends of the sheets.
However, there have recently been demands for increasing the capacity of the cassette and making the cassette universally suitable for different sheet sizes. It is thus necessary that the trailing end regulating plate satisfies the same function as that in a regular state regardless of the position of the trailing end regulating plate.
In technique (1), although the angle of the trailing end regulating plate 56 with respect to the intermediate plate 51 is determined by one side of the cut hole 51a for guiding the trailing end regulating plate 56, since the lower end of the trailing end regulating plate 56 is pivoted to the cassette case 55, the rotational axis 51b must be set at the lowermost position, i.e., the bottom, of the cassette case 55, to ensure an appropriate rotation range of the intermediate plate 51. However, as described above, in a large-capacity cassette, in order to prevent, as much as possible, forward and backward displacement of the front end of the intermediate plate 51, it is effective for maintaining the separation performance to set the rotational axis 51b of the intermediate plate 51 at substantially the vertical center of the cassette case 55. When the rotational fulcrum 51b is then set at substantially the vertical center of the cassette case 55, as shown in FIG. 14(b), the trailing end regulating plate 56 is pushed forward by one lengthwise side of the cut hole 51a and is therefore caused to be inclined by upward rotation of the intermediate plate 51. Thus, the sheets P are excessively pushed forward by the upper portion of the trailing end regulating plate 56, and the sheets P collide with the separation claws and interfere with the free vertical movement of the separation claws, thereby causing the possibility of poor feeding. In addition, the upward movement of the sheets P is interrupted by the collision with the front side of the cassette case 55, and the top sheet P idles without contacting the feeding rollers, thereby causing the possibility of no feeding.
Although conventional technique (2) is effective for the case where the trailing end regulating plate 57 is near the rotational axis 59a of the intermediate plate 59, when the trailing end regulating plate 57 is in front of the rotational axis 59a, e.g., when the sheets have a short size such as A5 size or the like, the trailing end regulating plate 57 is excessively moved forward from the sheet trailing ends. Namely, the trailing ends of the sheets are excessively pushed by the trailing end regulating plate 57, thereby causing the possibility of poor feeding or no feeding, as in conventional technique (1).
In order to solve the above problems, a degree of play may be permitted in the operation of the trailing end regulating plate. However, this brings about a degree of variation of the amount of engagement between the sheets and the separation claws, and the initial purpose of the trailing end regulating plate cannot be thus achieved.